In a rapid progress in lowering of price and miniaturization with the popularization of mobile communication devices such as cell phones, demand for lower price, smaller size, and lower profile are increasing of piezoelectric oscillators such as crystal oscillators. In response to such demands, not only packaging of crystal vibrators but also the integration of an oscillating circuit including a frequency adjustment circuit and a temperature-compensating circuit is made to reduce the number of parts and the size of devices.
FIG. 11(a) is an exploded perspective view of a conventional crystal oscillator as an example of conventional piezoelectric oscillators. FIG. 11(b) is a longitudinal cross-sectional view of the crystal oscillator.
This crystal oscillator 1 has a construction in which a crystal vibrator 11 is mounted on the top of a lower package 2 formed of ceramic laminations to integrate them into a single device.
The lower package 2 has an outer frame 3 raised along the edge so as to surround the inner part. An IC 6 is fixed to the inner side of the bottom in the depression 4 by means of a binder 5 such as solder, and the terminals 6a formed on the top side of the IC6 and the bonding pads 7 formed on the inner side of the bottom of the depression are connected by bonding wires 8. In the case of forming the terminals on the bottom side of the IC, the IC may be flip-chip-mounted by connecting the terminals of the IC with the bonding pads 7 by ball bonding. External terminals 9 for surface mounting are formed on the outer side of the bottom of the lower package 2, and the external terminals 9 are connected with the bonding pads 7 by means of conductors not shown in the figure. Top terminals 10 are formed on the top side of the outer frame 3, which are connected with the bonding pads 7. The depression 4 may be filled with an insulating resin to embed the IC 6 in a resin if necessary.
The crystal vibrator 11 has a construction in which a crystal vibrating element 14 is mounted in the depression 13 of a package 12 formed of ceramic laminations, and the depression 13 is hermetically sealed by a metal cover 15. Bottom terminals 16 are formed on the outer side of the bottom of the package 12. The bottom terminals 16 are bonded to the top terminals 10 of the lower package 2 by means of a binder such as a conductive adhesive, and the depression 4 of the lower package 2 is closed by the crystal vibrator 11.
This crystal vibrator 1 could be realized by integrating all circuit components making up an oscillating circuit and a temperature-compensating circuit except the crystal vibrator 11 into a single chip IC 6.
However, to produce the piezoelectric oscillator using the above-described expensive IC 6 with all circuit components integrated into it, it is a prerequisite that there is an expectation of a certain amount of quantity production of the piezoelectric oscillator 1 itself. Without an expectation of quantity production, cost reduction is difficult. Since it is therefore impossible costwise to use this type of expensive IC 6 for piezoelectric oscillators manufactured in large item small volume production, for example. Hence the circuit components for making up the above-described circuits such as transistors, resistors and capacitors, instead of the IC 6, must be mounted in the depression 4 of the lower package 2 in the form of separate chip-shaped parts.
When attempting to mount such chip-shaped circuit components in the depression 4 of the lower package 2 shown FIG. 11, they must be mounted on the lands (formed for mounting chip-shaped circuit components instead of the bonding pads 7) by reflow soldering using cream solder. Application of cream solder to the lands is usually carried out by screen printing using silk screens (masks). However, in the case of the lower package 2 where there is an elevated part, outer frame 3, around the surface on which the lands are formed, the silk-screen printing technique cannot be used. It is therefore difficult to increase the productivity by a batch process using the silk-screen printing, and application of cream solder to the lands in the depression 4 must be carried out one by one by a dispenser. This causes an increase in cost if chip-shaped parts are used.
For this reason, in the case of constructing a piezoelectric oscillator using discrete chip-shaped parts, there is no choice but to use a construction in which lands are formed on the top side of a large-area circuit board and chip-shaped parts 21 and a crystal vibrator 22 are mounted on the lands by reflow soldering as shown in FIG. 12. Since the circuit board 20 of this construction is flat without differences in level and hence the silk-screen printing can be used to apply cream solder to the lands formed on it, a batch process using a large circuit board block, which consists of a large number of the circuit boards for the piezoelectric oscillators arranged side by side in the vertical and horizontal directions, can be used.
However, since all components 21 and 22 are mounted on a flat circuit board, this construction consequently needs a larger board occupation area, bringing a result against the demand for parts suitable for high density mounting.
A purpose of the present invention is therefore to provide a piezoelectric oscillator which can use a flat circuit board to mount circuit components, making it possible to apply cream solder to the lands formed on the circuit board in a batch process for mass production and whose board occupation area can be reduced by three-dimensionally arranging a packaged piezoelectric vibrator above the circuit board.
Another purpose of the present invention is to provide a method of manufacturing the piezoelectric oscillator which makes efficient production using a batch process possible.